Programatically Comparing Versions Using Chef's Versioning Schemes

Chef's got some pretty great ways of definining version constraints of its dependencies, which can be used across cookbook dependencies, gem dependencies and supported platforms.

Today, I wanted to write some code that checked whether a cookbook supports RHEL7, but not RHEL8.

To do this, we would have our metadata.rb:

# not great
supports 'redhat', '< 8'
# better
supports 'redhat', '~> 7.0'

But how do we then determine this programatically? We can use Chef::VersionConstraint::Platform to parse the supported value, like so, and then compare it against a specific version:

require 'chef'

m = Chef::Cookbook::Metadata.new
m.from_file 'metadata.rb'
constraint = Chef::VersionConstraint::Platform.new m.platforms['redhat']
puts constraint.include? '6'
# => true
puts constraint.include? '7'
# => true
puts constraint.include? '7.5'
# => true
puts constraint.include? '7.5.0'
# => true
puts constraint.include? '8'
# => false

If you're comparing cookbook versions, you can just use Chef::VersionConstraint.new.